What is the best way to contact OVO customer service / support team?

If you were not happy with what OVO had offered then you should have gone back to the ombudsman and added that fact to your case.
(I had to do that myself several times when another supplier would not do as directed by the ombudsman),

However you say that you accepted the OVO offer.
Having accepted the offer you can’t really just change your mind later.

Right now the app doesnt even match the readings they have to me privately, the app shows estimates as given readings,

Any estimated usage will get remedied when actual meter readings are given, either by a smart meter or by you sending OVO a meter reading.

If a supplier isn’t getting any meter readings from either a smart meter or the customer then all they can do is estimate bills.

As it appears that your meter isn’t providing readings for some reason then you need to send them in yourself if you want accurate bills and not estimates.
How often are you sending in meter readings?

I was told ovo would use an estimate, which I agreed to do, I just wasnt told it would be an absurd estimate that would be higher than my average usage.

When I went back to the Ombudsman I was told as I'd accepted the offer and there was nothing I can do. So I say again, to anyone reading, DON'T accept estimates when ovo don't provide the actual bill. Live and learn,.some companies are evil and those estimates will be deliberately wrong.

"If a supplier isn’t getting any meter readings from either a smart meter or the customer then all they can do is estimate bills."

And thoae estimates should be reasonable,  and when a customer provides evidence that they hadn't moved in, that evidence should have an effect on said estimate.

The updated readings were provided after the original meter was located and the final reading taken months ago, the app STILL doesnt match the pdfs given to me by email. That's months after. And this still doesnt resolve the fact nobody has replied.

On the app every reading has to have stated that it was provided by customer or smart meter etc, and there's places thst information is incorrect. 

This isnt a problem with my meter sending readings. It's a problem with Ovo. I have no interest in attempting to resolve this with ovo anymore,  I just wanted to warn other customers to avoid this joke of a company.